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Author Topic: L78 stalls when hard braking  (Read 2873 times)
babaron
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« on: September 22, 2010, 06:12:57 PM »

I'm finding that my L78 is stalling during hard braking even with the clutch depressed.  Is this common or something that needs to be fixed?
THX
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JohnZ
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« Reply #1 on: September 23, 2010, 10:12:00 AM »

I'm finding that my L78 is stalling during hard braking even with the clutch depressed.  Is this common or something that needs to be fixed?
THX

Check your float levels. If the secondary float is too high, hard braking can result in fuel coming out of the rear bowl vent tube and dumping into the secondary. If the primary float is too low, hard braking can result in fuel uncovering the primary jets so they suck air instead of fuel.
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'69 Z/28
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babaron
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« Reply #2 on: September 24, 2010, 06:13:14 PM »

Thanks for thr help, John.  I'll check that out.  One more question: I've heard serveral people tell me I will need to have the carb rebuilt every once in a while especially if it is not driven regularly.  Would you wait until there is a problem with it or have it rebuilt periodically to keep it running at peak performance?

Ron
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My68SS
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« Reply #3 on: September 25, 2010, 08:30:13 AM »

From my experience, the only thing that will suffer in a carby that sits idle for long periods is the rubber diaphrams in the accelerator pumps and any O-rings that are normally exposed to fuel.
Run the engine to temp once a week or so and that will keep them immersed in fuel.

But beware of acid build-up in your engine oil though, especially from repeated short runs of the motor. You may want to change that on an even more regular basis.
In fact, acidification of the engine oil is of much more concern than the carby.
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Rob
1968 L34/M40 SS
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JohnZ
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« Reply #4 on: September 25, 2010, 11:00:45 AM »

Thanks for thr help, John.  I'll check that out.  One more question: I've heard serveral people tell me I will need to have the carb rebuilt every once in a while especially if it is not driven regularly.  Would you wait until there is a problem with it or have it rebuilt periodically to keep it running at peak performance?

Ron

I'd leave it alone - "If it ain't broke, don't fix it". My cars only get a couple of hundred miles a year and sit untouched for six months every winter, and I haven't touched the carburetors for 8-9 years; they run great.
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'69 Z/28
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tmodel66
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« Reply #5 on: September 25, 2010, 11:10:11 AM »

What kind of fuel stabilizer do you use? Do you try to keep your tank full or almost full?
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Daniel  
'69 SS 350/4 speed  Fathom Green--POP
JohnZ
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« Reply #6 on: September 26, 2010, 11:32:44 AM »

What kind of fuel stabilizer do you use? Do you try to keep your tank full or almost full?

I've never used any kind of fuel stabiliizer; when I put the cars away for the winter, I fill the fuel tanks and change the oil the day before, and leave them like that. Have been doing that for 40+ years. In the spring, I fill the float bowls through the bowl vent tubes, and they fire right up.
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copo69
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« Reply #7 on: September 26, 2010, 03:00:58 PM »

John, do you change the oil again in the spring before driving?
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babaron
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« Reply #8 on: September 26, 2010, 09:02:29 PM »

I just bought a fuel stabilizer.  I'm going to use some at next fill-up.  What is the recommendation on where to keep the fuel guage? Should I keep it almost full all the time?  How often should I change the oil if I drive it once every week or two?  Thanks!
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JohnZ
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« Reply #9 on: September 27, 2010, 10:06:27 AM »

John, do you change the oil again in the spring before driving?

Nope - the clock doesn't run on motor oil while the car's not being used and the oil is fresh; that's why I change it just prior to putting the car away for the winter, and I don't start it at all during that time. If you're not going to drive the car at least ten miles (to get the oil hot enough to boil off the rich blow-by contaminants and condensed moisture, which can make the oil acidic), don't start it at all during storage; the seals won't "dry out" - that's an old wives' tale.
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JohnZ
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« Reply #10 on: September 27, 2010, 10:11:08 AM »

I just bought a fuel stabilizer.  I'm going to use some at next fill-up.  What is the recommendation on where to keep the fuel guage? Should I keep it almost full all the time?  How often should I change the oil if I drive it once every week or two?  Thanks!

If you're talking about winter storage, fill the tank first. I change the oil and filter once a season, just prior to storage.
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JoeC
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« Reply #11 on: September 28, 2010, 08:22:29 AM »

The old style Holley bowl gaskets will leak sometimes just from sitting but the newer style blue gaskets seem to be a lot better.

always check for fuel leaks on Holleys !
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babaron
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1969 L78 convertble


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« Reply #12 on: September 29, 2010, 04:51:10 PM »

Joe, this Holley definitely leaks gas! Other issues include missing choke and plating shot.  I am thinking about having JM rebuild it for $330. It runs OK but definitely leaks gas. Would you just change the gaskets or rebuild? How serious is a gas leak?  Are we talking high risk of fire or just bad smell?

thanks!

Ron
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Gramps69Z
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« Reply #13 on: September 29, 2010, 08:19:29 PM »

  How serious is a gas leak?  Are we talking high risk of fire or just bad smell?

thanks!

Ron

Any gasoline leak is high risk.
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Captain John Wykoff
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My68SS
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« Reply #14 on: October 01, 2010, 08:17:51 AM »

I'll second the above risk comment! Whilst a slow weep could be considered low risk, you only know that while you've got the hood open and are looking at it.
What might happen after you shut the hood and start driving is now unknown.
The moment you smell/see fuel leaking - you gotta fix it.

I should modify my earlier comment about accelerator pumps by saying that it's only those carbys that have plunger type pumps like rochesters that suffer from shrinkage of the cup after sitting dry for a while and initially don't pump so well, but usually recover after a few days from when the engine is first run.
Holley's with the diaphram pump don't suffer this problem.
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Rob
1968 L34/M40 SS
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red67l78
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« Reply #15 on: October 30, 2010, 09:43:47 AM »

Personally i start my car up at least one time throught the winter time. Wait for a warmer day and start car up. let run till nice and warm. This helps battery and charging system componets. Also trans fluid runs throught trans. Water pump turns also. Valve train is changed location. Rubber diaphrams throught out car are moved, distributor ,power brakes , carb power valves.fuel pump, auto trans too. Basically move things that are sitting idle over the winter. Oh ya. I pump my brakes at least 3-4 times over the winter to move diaphrams there to.  Please keep fire extimgusher near car and easy to find.
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Charley
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« Reply #16 on: October 30, 2010, 11:23:37 AM »

Alot of times they might leak because of something as simple as loose screws. Can you see where it is leaking from ? If you gently tighten the 4 float bowl screws on each end of the carb you might stop the leak.
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red67l78
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« Reply #17 on: October 30, 2010, 12:40:25 PM »

Hey Bar give us alittle more info on problem.  These engines have very little vaccume @ idle.  So idle needs to be healthy. Do you have power brakes?  carb size and stock intake?  Do you down shift to slow car?
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babaron
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1969 L78 convertble


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« Reply #18 on: October 30, 2010, 03:08:23 PM »

This car has the stock pwr front disc brakes and the original 4346 Holley carb.  I try not to downshift a lot but do a little now and then. This problem may be a moot point now that I've sent it off to be rebuilt. I expect it back in a week or two and will update you all on how it works. Thanks!
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red67l78
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« Reply #19 on: October 30, 2010, 07:21:20 PM »

Get info from rebuilder about what jets and power valves are in it and what he is putting in. Ask him if float settings were off. You may need this info later. Get vacumme reading when you reinstall carb.  Check one way valve in brake booster.
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